1. Field of the Invention
This invention relates to an improvement in a dielectric drying process for honeycomb structures.
2. Related Art Statement
Heretofore, the dielectric drying process has been carried out in order to dry the honeycomb structure of a ceramic green structural body obtained by extruding a ceramic material through a die and having many parallel through-holes isolated from each other by partition members each having an approximately uniform wall thickness. That is, the honeycomb structure was set between opposed electrodes and then an electric current was applied across the electrodes to conduct molecular motion of dipolar of water in the inside of the honeycomb structure through the generated high frequency energy, during which the honeycomb structure was dried by the friction heat accompanied therewith.
However, when the honeycomb structure is dried by the above dielectric drying process, there is caused a drawback in that the density of the electric force line passing through the honeycomb structure becomes non-uniform. In order to solve this drawback, the inventor has previously proposed, in U.S. Pat. No. 4,439,929, a drying support board composed of a perforated plate, a predetermined region of which, inclusive of a portion contacting with a lower opening end face of the honeycomb structure, has a conductivity higher than that of the other remaining peripheral portion thereof.
When the honeycomb structure is subjected to a dielectric drying by using the above drying support board, the density distribution of the electric force line becomes uniform to a certain extent, but the density in the upper portion of the honeycomb structure is still non-uniform, and consequently the drying of the upper portion in the honeycomb structure becomes slow as compared with the other remaining portion. That is, the drying shrinkage in the dry-delaying portion is small as compared with that of other portion, so that the dimensional scattering is caused between the upper portion and the lower portion in the honeycomb structure after the dielectric drying and hence the dimensional accuracy lowers. As a result, the size of the upper portion becomes undesirably larger than that of the lower portion.
Furthermore, when the drying of the upper portion in the honeycomb structure is delayed to form a high-moisture region in this upper portion, if the draft drying or firing is carried out after the dielectric drying, the shrinkage becomes large only in the highmoisture region and cracks are apt to be caused.
Therefore, the following has been provided which is capable of uniformly subjecting the honeycomb structure to dielectric drying without delaying the drying of the honeycomb structure as a whole.